1. Field
The present disclosure relates to methods and apparatus for estimating a sleep clock frequency and, more particularly, estimating a sleep clock frequency using a combiner to selectively determine an estimate of the sleep clock frequency.
2. Background
Mobile wireless devices, such as mobile phones, typically employ clock that provides very accurate timing for various functions within the device including keeping the system time. These clocks many times are embodied using temperature controlled crystal oscillators (TCXOs). TCXOs, however, use a relatively large amount of power, drawing approximately 1.5 mA of current.
In order to improve the battery life of a mobile device, it is known to place most current consuming units within the device into a power saving mode and maintain the system time using low-power sleep circuits. Because of the high current draw of TCXOs, it is not energy efficient to use such devices to maintain system time for sleep circuits. Accordingly, it is known to maintain system timing during sleep or power saving modes using by using a sleep controller having a much lower power usage (e.g., a clock with a current draw of 200 μA) and a lower frequency (e.g., 30-60 kHz) than TCXO devices, which operate a higher frequency (e.g., 44-66 MHz). This is typically accomplished with a cost effective crystal oscillator clock at the expense of some accuracy in time keeping because the clock frequency tends to fluctuate. This clock is otherwise known as the “sleep clock” or “slow clock.” Thus, when the mobile device is asleep, the TCXO (or “fast clock”) is off. The sleep clock is used as a timer to wake up the system. Upon wake up, once the fast clock becomes stable after waking up, system timing is once again handed over to the fast clock.
It is noted that when a mobile device wakes up from a sleep mode, it is important to have an accurate system time as kept by the sleep clock. Since the sleep clock is used for system timing during sleep modes, the accuracy of the clock timing will directly affect the system time when the mobile transceiver wakes up prior to re-acquisition of timing based on information received from the wireless network, such as a CDMA based network. A good estimate of slow clock frequency is therefore desirable. Known timing estimation utilized by mobile devices, however, is typically used only for initial calibration and the slow clock time tracking is solely dependent on Pseudo Noise (PN) code timing. In certain wireless systems not employing PN timing (e.g., Orthogonal Frequency Division Multiplexing (OFDM)), however, this timing in not available. Thus, in such systems the accuracy of the sleep clock timing is even more important. In the case of OFDM, in particular, such systems are more susceptible to timing errors such as synchronization timing made worse by intersymbol interference.
It is further noted that issues exist related to determining an accurate estimate of the slow clock. First, the resolution of sleep clocks is low and the sleep clock is not synchronized with the fast clock. This issue can be resolved, however, by using the fast clock (derived from TCXO) to count the fractional part of the sleep clock as detailed, for example, in the co-pending U.S. Patent Application entitled “APPARATUS AND METHODS FOR ESTIMATING A SLEEP CLOCK FREQUENCY” referenced above and expressly incorporated by reference.
Furthermore, because the slow clock is a low accuracy, free running crystal without temperature compensation, the system time may drift during sleep. Accordingly, it is desirable to account for this drift and thus determine a maximum sleep time that may be set for the mobile device so that there is no need for re-acquisition of system time on wakeup. Finally, because the sleep clock is used as a timer during sleep, it is necessary to have a good estimate of the sleep clock frequency before going to sleep.
It is possible using the methodologies above to achieve an accurate estimate of the sleep clock frequency for specific situations. However, in the case of OFDM, for example, determining system timing using CDMA PN codes may not be available for determining timing. Moreover, determining an estimate of the sleep clock using the fast clock may yield a very accurate estimate of the sleep clock frequency, but does not account for the change or error due to drift of the sleep clock. Furthermore, if the sleep clock frequency is accurately determined and the error or drift of the sleep frequency is determined, how to combine this information to yield an accurate result has not been contemplated in the known art.